You get an upgrade half way through the campaign that gives you +30 seconds on secret levels you have already completed. Just play until you can unlock that, and then play the secret levels, you will have to play them once, finish them, and then play again, the 30 second boost will be enough to get gold due to the score boost in almost every level, Even the "Hardest" one.

Some of them you can get gold without the 30 second boost if you finish them without getting hit and get most of the points along the way.

Don't do what I did and waste 3 hours trying to get a perfect run on the secret levels to gold medal them as you unlocked them, just wait until you get the 30 seconds upgrade.